1. Field of the Invention
The field of the invention relates generally to the aeration of liquids. More particularly, it relates to the aeration of liquids by passage of gas bubbles through a tube which is vertically submerged in the liquid and within which tube the rising gas bubbles are broken up by mechanical action into very much smaller gas bubbles which are dispersed within the liquid.
2. Description of the Prior Art
Aeration of liquids is commonly performed, for example, to accelerate bacteriological decomposition of liquid wastes, to prevent algae formation on the surfaces of stagnant pools or ponds, etc. The term "aeration" as employed herein is to be understood as denoting the introduction of any type of gas into any type of liquid.
The simplest method of aeration comprises introducing a gas into a liquid through holes in an appropriate supply line. Some of this gas is absorbed as the gas bubbles rise through the liquid. Unabsorbed gas escapes from the surface of the liquid, and may or may not be captured for recirculation.
This method, though simple, is very inefficient. The gas bubbles, even if small when introduced into the liquid, tend, as they rise, to aggregate into large bubbles or slugs of gas. These gas slugs have comparatively small surface area to volume ratios. That is, there is relatively little gas-to-liquid contact, considering the volumes of the gas bubbles. This results in relatively low rates of gas absorption by the liquid at the liquid-gas interfaces. If the openings at the gas outlet are made very small to introduce small gas bubbles, fouling of the openings often occurs. Also, the transit time of the gas through the liquid may be quite short if the liquid container, for example a pond or holding tank, is shallow. This short gas-to-liquid contact time further results in an inefficient rate of gas absorption by the liquid. In addition, minimal turbulence is created for disrupting the liquid-gas interfaces, disruption and renewal of the interfaces being essential for high rates of gas absorption.
Some slight improvement in absorption efficiency is obtained by use, at the gas injection openings, of nozzles which introduce the gas into the liquid in a swirling manner so as to create some degree of turbulence. This also tends somewhat to delay formation of large gas slugs and somewhat to disperse the gas bubbles through a larger volume of liquid (for example, U.S. Pat. No. 3,276,698). High absorption efficiencies are still not obtained, however.
More commonly used processes employ the pneumatic (or air) lift pump principle. When a gas is bubbled up through an elongate tube which is vertically submerged in a liquid, the rising gas bubbles cause an upward lifting or flow of liquid through the tube. This upward flow of liquid causes a circulation within the entire body of liquid, liquid being continually drawn into the bottom of the tube and being discharged from the top thereof. Turbulence in the liquid above the top of the tube (which is normally submerged well below the surface of the liquid) tends to improve the absorption rate of the gas by breaking up, to some extent, large gas slugs (for example, U.S. Pat. No. 3,032,496) and by disrupting and renewing the liquid-gas interfaces. The liquid circulation and turbulence caused by such pneumatic lifts may also be used to prevent formation of ice on the surface of a liquid, or to reduce the magnitude of surface waves, for example in a harbor area. The absorption efficiency obtained is still much less than desired, however, because large gas slugs still tend to form and remain unbroken, and because the gas-liquid contact time is not appreciably increased. Therefore, a considerable amount of gas must be pumped through such pneumatic lift tubes in order that a small amount may be absorbed by the liquid. Because of the inefficient absorption process, much of the energy used to pump the gas is wasted.
Helical tube dividers installed in some pneumatic lift tubes (for example, U.S. Pat. Nos. 1,144,342 and 3,452,966) increase the gas-liquid contact time by providing increased path lengths for the gas bubbles to travel as they spiral up through the tubes. In addition, the gas and liquid exits from the tops of the tubes with a rotational motion, thereby somewhat increasing the turbulence thereabove. However, large slugs of gas still tend to form within the tubes, with consequent still relatively poor absorption efficiency. Some helical tube dividers (for example, U.S. Pat. No. 1,144,342 are provided with holes interconnecting the adjacent chambers to help prevent formation of large gas slugs. There is little tendency to produce small gas bubbles and the gas absorption efficiency is still much less than desired. Gas which is not absorbed in the bubble transit through the liquid is either lost or must be repumped through the liquid. This requires additional gas pumping capacity and horsepower.
Because of inefficiencies of present pneumatic lift tube aerators, it has been necessary to pump relatively large amounts of gas through the liquid -- only a relatively small portion actually being absorbed by the liquid -- and to employ a relatively large number of pneumatic lift tubes, particularly when the liquid is contained in shallow tanks or ponds and short tubes must be used. Thus, there has been considerable wastage of gas pumping power with resulting high costs involved in such complex aerator systems.
Some aerators include a motor-driven, horizontally rotating submerged turbine. The non-enclosed turbine is generally positioned above a source of gas bubbles and is used to break up and disperse the released gas bubbles and to create turbulence in the liquid. Other aerators employ motor driven, vertically rotating, non enclosed turbines or paddles at, or just below, the surface of a liquid. Such aerators usually rely upon the air above the surface of the liquid, some of which becomes entrapped in the churning liquid, for aeration. However, motor-driven aeration systems are expensive to produce, to operate and to maintain. A source of power for the motor must also be available.
There have heretofore, to the applicant's knowledge, been no simple, economical, reliable means to achieve efficient aeration with a minimum expenditure of gas pumping horsepower and without the need for auxilliary driving sources for mechanical rotor elements.